The invention relates to a high-pressure gas discharge lamp comprising:
a lamp vessel which is closed in a vacuumtight manner and has a quartz glass wall enclosing a discharge space;
metal foils embedded in the wall of the lamp vessel and each connected to a respective external current conductor;
tungsten electrode rods each connected to a respective one of said metal foils and projecting from the wall of the lamp vessel into the discharge space;
an ionizable filling in the discharge space;
the lamp being defined by the following relation
finw greater than =40%
xe2x80x83in which:
finw=fraction of length of the electrode rod enclosed in the wall of the lamp vessel.
A high-pressure gas discharge lamp of this type is known from U.S. Pat. No. 5,461,277. The known lamp is suitable for use as a vehicle headlamp and has electrode rods of a thickness of 250 xcexcm which may or may not have an envelope at their ends and may be made of, for example, thoriated tungsten.
Stringent requirements are imposed on the speed with which the lamp, after it has been ignited, provides a large fraction of the luminous flux during stable operation. It is also necessary that the lamp can be ignited while it is still hot due to a previous operating period. The lamp is ignited at a voltage of several kV and a frequency of several kHz in order to comply with these requirements.
In the manufacture of the known lamp, a seal is made in which one or several of said metal foils are enclosed in the wall. During this operation, the quartz glass is softened at the area where this seal is to be created in the presence of the metal foil, the external current conductor and the electrode rod. Subsequently, the lamp, or the lamp-to-be, cools down. Due to its relatively high coefficient of linear thermal expansion (approximately 45*10xe2x88x927 Kxe2x88x921), the electrode rod then contracts more strongly than the quartz glass in which it is embedded. Quartz glass is a glass having an SiO2 content of at least 98% by weight, the coefficient of expansion of the glass is approximately 6*10xe2x88x927 Kxe2x88x921. For a good adhesion between the rod and the quartz glass, obtained by an additive to the electrode rod tungsten, such as thorium oxide, a coating of quartz glass around the rod is obtained, which is mechanically unconnected with the quartz glass of the wall. If the electrode rod and the quartz glass adhere insufficiently to each other, a capillary space is created due to shrinkage around this rod. No such capillary space is created around the metal foil, often a molybdenum foil, because of the foil shape.
In the known lamp, there is often a good adhesion between the rod and the quartz glass and thus there is a coating of quartz glass around the rod. The quartz glass coating of the electrode rods in the known lamp enhances their thermal capacity (the energy which is necessary for the same rise of temperature) and also increases their thermal conductance (the quantity of heat which can be depleted per unit of time). On the other hand, their electrical conductivity is not affected. The higher thermal capacity retards the rise of temperature of the rods during ignition of the lamp, so that the permanent contact with the embedded metal foil enables the surrounding quartz glass of the wall to assume a higher temperature and to expand, also because of the heat developed in this foil due to the passage of current.
It has been found that the coatings of species of one type of lamp may have alternating lengths. This may be due to small variations of temperature of the quartz glass when the seal is being made. It is a drawback that the absence of a coating or an insufficient coating results in rejects during the lamp production and that the known lamp has only a short lifetime when there is no or not enough quartz glass coating and when this lamp is often switched on and switched off after a short operating period.
When such a lamp without coating is ignited, the temperature of the electrode rods rises steeply owing to the high current flowing through them and owing to heat transfer from the discharge. The quartz glass does not instantaneously follow this temperature rise. Owing to their higher temperature and their higher coefficient of expansion, the rods will come into contact with the quartz glass and exert pressure on it. It was found that damage, such as microcracks, then occurred in the quartz glass, which microcracks generally increase in number and size during subsequent ignition periods. This leads to a (premature) end of the lifetime of the lamp owing to leakage, causing constituents of the filling to escape so that the lamp no longer ignites, or the lamp vessel is broken.
Lamps complying with the relation finw greater than =40% have a greater risk of occurrence of the above-mentioned detrimental phenomena, unless special circumstances are created, for example, a quartz glass coating around the electrode rod.
Another drawback is that the coating leads to unwanted and troublesome reflections of the light generated in the discharge.
U.S. Pat. No. 5,510,675-A discloses electrodes, a part of which is made of rhenium and has a thickness of 400 xcexcm. However, the part made of rhenium does project very far into the discharge space and is only provided with a head at its very last end, with a thickness of, for example 1 mm or an enveloping winding of tungsten. However, this large head leads to the unfavorable effect of lamp flickering, i.e. the point of contact of the discharge arc displaces suddenly over the head.
It is an object of the invention to provide a high-pressure gas discharge lamp, having a simple construction and counteracting said drawbacks.
According to the invention, the electrode rods have first parts projecting into the discharge space, which first parts are at least substantially made of tungsten, and second parts enclosed at least partly in the wall, which second parts have a thickness ranging between 250 xcexcm and 350 xcexcm and at least an envelope of rhenium, said first and second parts contacting and being connected to each other via facing ends.
Since the electrodes are composed of a first and a second part, it is possible to adapt the electrodes to circumstances. The first part is made in conformity with the end of the electrode of the known lamp projecting into the discharge space, so that it can withstand the heat developed by the high starting currents and the discharge during the lifetime of the lamp. The first part of the electrode is made of tungsten so that a strong evaporation of electrode material is prevented, as will occur if the first part consists of rhenium. The second part is designed in such a way that the problem of leakage or breakage of the lamp due to expansion and, consequently, exertion of pressure on the quartz glass by the electrode rod upon (re)ignition of the lamp at least substantially does not occur anymore. The first and the second part of the electrode may be secured to each other in accordance with conventional techniques such as laser welding.
In second parts having a relatively thick envelope of rhenium or being entirely made of rhenium, a greater thickness is necessary than when this second part is made of, for example, tungsten having a relatively thin envelope of rhenium due to the smaller coefficient of thermal conductance of rhenium compared with tungsten, SRexcx9c0.3*SW. Experiments have proved that a thickness of minimally 250 xcexcm is necessary for second parts substantially made of rhenium so as to ensure a sufficient depletion of heat.
It has been found that in lamps complying with the relation finw greater than =40%, the occurring problems of leakage at least substantially do not occur at relatively small thicknesses of second parts of the electrode rods enclosed in the wall. The risk of leakage or breakage of the lamp is considerably reduced if the thickness of these second parts is chosen to be smaller than 350 xcexcm. The successful use of relatively large thicknesses in second parts having at least an envelope of rhenium is based on the ductility of rhenium. When exerting pressure on the quartz glass due to expansion by the electrodes, this pressure will be more evenly distributed due to deformation of the relatively ductile rhenium than when using electrodes which are made of, for example, the much less ductile tungsten. If electrodes having at least an envelope of rhenium are used, fewer concentrations of tensions will thus occur in the quartz glass so that the use of larger thicknesses than in similar tungsten electrodes is possible.
An important advantage of the measure according to the invention is that it provides the possibility of using electrode rod material which is free from thorium without detrimentally influencing the lifetime of the lamp.
The capillary spaces which have formed during embedding of the electrode rod in the quartz glass are relatively small in second parts having thicknesses of less than 350 xcexcm. Therefore, no large quantities of salts can accumulate in these capillary spaces, which salts would otherwise have been extracted from the discharge. In the relatively small capillary spaces, the second part of the electrode rod locally makes permanent contact with the wall of the lamp vessel so that a satisfactory depletion of heat is obtained.
Due to the relatively small thermal conductance in second parts which are largely or entirely made of rhenium, it is also favorable that the first parts proximate to their transition to the second parts are in permanent contact with the wall of the lamp vessel, for example, partly enclosed in the wall, for example, over a length of 0.1-1.0 mm. The depletion of heat of the composite electrode is thereby even further enhanced.
Due to the high starting currents upon ignition of the lamp and the heat developed as a result of the discharge, not only relatively high temperatures occur in the second parts but also in the first parts of the electrodes. In first parts having a thickness of less than 250 xcexcm, there is a relatively great risk of melting. Electrodes having first parts with a thickness of more than 250 xcexcm have a sufficient thermal conductance so that the risk of melting is reduced quite considerably. Moreover, the first parts preferably have a thickness of less than 400 xcexcm. Then there is hardly any risk that the unfavorable effect of lamp flickering will occur.
The lengths of the first and second parts are also determined by the total length of the entire electrode. In a favorable embodiment the entire electrode has a length of 4.5 to 7.5 mm, preferably 6 mm. The choice of the length of the separate parts is such that the connection of the first part to the second part is at least substantially located at the boundary surface of the wall and the discharge space, at the location where the electrode projects into the discharge space.
The high-pressure gas discharge lamp according to the invention may be used, for example, as a vehicle headlamp or in an optical system of a different kind. To this end, the lamp may be provided with a lamp cap and may or may not be surrounded by an outer envelope. A lamp cap may or may not be integrated with a reflector.
The metal foils may be embedded next to one another in one region of the wall, or in regions situated at a distance from one another, for example, opposite one another. The first parts of the electrode rods may or may not have an enveloping winding at their ends in the discharge space. The first parts of the electrode rods may be made of undoped tungsten, for example tungsten-ZG, or of doped tungsten such as W with 1.5% by weight of Th.
When doped tungsten is used, a small content of crystal growth-regulating means such as 0.01% by weight in total of K, Al and Si may be added so as to influence the tungsten grain size. The second parts may be made of undoped rhenium or of doped rhenium, for example, doped with Mo and/or W, normally with a doping concentration of less than 10% by weight in total.
The ionizable filling may comprise, inter alia, a rare gas, mercury and a mixture of metal halides, for example, rare-earth halides which are the halides of the lanthanides, scandium and yttrium.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.